Volume-control circuits



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VOLUME CONTROL CIRCUITS Original Filed Aug. 9, 1950 PATENT OFFICE VOLUME-CONTROL CIRCUITS Charles T. Jacobs, New Providence Township,v Union County, N. J.

Reled for abandoned lapplication Serial No. 474,272, August 9, 1930. This application April 16 Claims.

This invention relates to electrical systems for sound production or reproduction, and more particularly to methods and apparatus for controlling the volume of sound output of such systems. This application is a substitute for my prior application, Serial Number 474,272, filed August 9th, 1930.

It is well known that the ear is differentially sensitive to sounds of various frequencies-i. e., 10 that it less readily perceives changes in the amplitude of sound at certain frequencies than at others. It is most sensitive to sound amplitude changes in the extreme (very high and very low) audible registers, and least sensitive to changes at intermediate frequencies. If in systems of the class described the general amplitude, or level, of the sound output be changed by any operation which uniformly alfects all the diierent frequency components, it will seem to the ear that the amplitudes of components of extreme frequencies have been changed to a greater degree than those of intermediate frequency. It is a broad object of my invention to provide means for so changing the sound output level of a system of the class describedv that the ear will perceive the amplitude changes at different frequencies as essentially similar. It is an allied object to provide, in association with means operative to control the sound output level, means for counteracting or compensating for the differential sensitivity of the ear abovementioned.

The most common method of changing the output sound level of this type of system is by the use of a potentiometer, across the entire resistance of which are applied electric oscillations conforming in their various characteristics to the eventual sound, the voltage thereby developed between the inferior extremity of the potentiometer and a contact selectively position- 4o able along its resistance being taken oi and translated into sound. I am aware that at least two circuit arrangements have been proposed in connection with such potentiometers for fulfilling the broad object of my invention. In one of these, as shown in U. S. Patent 1,785,048 to Reynolds, a variable frequency-discriminating circuit electrically independent of the potentiometer is employed, its control being mechanically linked to that of the potentiometer to operate in unison therewith; in the other, as shown in U. S.

Patent 1,788,035 to Stevenson, one or more' parallel L-C circuits are provided as a shunt between the superior extremity and the variable contact of the potentiometer. I have found, however, that the desired results may be obtained by other arrangements having great advantages in respect of effectiveness, simplicity, economy, and freedom from various undesirable effects, including susceptibility to external stray fields, etc. Accordingly it is a specic object of Serial N0. 606,891

my invention to provide improved circuits useful with potentiometer volume controls for fullling the broader objects of my invention.

It is a further broad object to provide means for simultaneously varying both the 7tone quality and volume level of the sound outputof a system of the class described, to effect predetermined relationships of quality to volume level. Other and allied objects will more fully appear from the following description and the appended claims.

In such description reference is had to the accompanying drawing, of which Figure 1 is a diagrammatic view illustrating my invention in one form; 75

Figure 2 is a like view illustrating my invention in a modified and in general preferred form;

Figure 3 comprises a set or family of curves showing certain effects of the use of my invention under certain typical conditions; and

Figure 4 is a diagrammatic view illustrating my invention in the form shown in Figure 2, but with the omission of one of the elements shown in that figure.

For purposes of most useful and appropriate illustration, but not intending any limitation thereby, I have shown and described my invention applied to a system which is presumed to produce a satisfactory tone quality when the control is set for the production of the maximum volume level. A problem is thus presented of reducing the volume level without appreciable disturbance of the amplitude ratios, as perceived by the ear, between all the different frequency components of the output. The solution of this problem 95 requires that, as general Volume level is reduced,

a greater reduction of oscillation amplitude take place at intermediate than at extreme frequencies.

An embodiment of my invention whereby this effect is produced is shown in Figure 1, in which P is a potentiometer having total resistance R1, variable contact or slider S, and variable portion R2 of resistance between slider S and the inferior extremity of the potentiometer. The contact or 105 slider S has been shown as continuously movable over the resistance R1, the word continuously o f course being used in the sense of denoting substantially unrestricted interpolation of contact positionings rather than in any sense of sustained 110 motion over a period of time. V1 is an electric cir- Quit element, shown schematically as a generator having output impedance Ro. It will be understood, of course, that V1 is intended to represent a plural frequency oscillation source; by way of ex- 115 ample, it may be a magnetic phonograph pick-up having output impedance Ro. The potentiometer P, shunted by the capacity C2, is connected in the output circuit of element V1, in series with resistance R3 and reactance comprising inductance 120 2 Lacasse L1 and capacity C1. The a. c. voltage appearing across C1, L1, Rs and the variable R2 in series is applied to the input of a succeeding circuit element Vz, shown for example as an electric vacuum tube having an input circuit comprising grid G2, cathode F2 and such input biasing potential source Bz as may be required. To the output of Vz may be connected such succeeding circuits as may be desired, which may terminate in an electro-acoustic translating device. Such circuits and translating device are shown in Figure l as a transformer and loudspeaker, respectively. Of the reaotances, which with the potentiometer and R3 comprise the control system proper, C1 of course increases the impedance of the lower portion of the system at lower frequencies; L1 increases it at the higher frequencies; and C2 decreases the impedance of the upper portion of the system at the higher frequencies.

The operation of this embodiment of my invention may be best understoodv quantitatively as well as qualitatively by recourse to the simple algebra of complex quantities. E1 may denote the no-load a. c. voltage output of V1 at any given to each other.

frequency w/21r, and E2 the voltage thereby produced at the input of V2 with any given setting of slider S. The ratio of Ez to E1 will then express the degree of amplitude control exerted on electric oscillations of such frequency by the control system with such slider setting; it may otherwise be termed the transmission efficiency of the control system for such frequency and adjustment. Provided the input impedance of V2 be very high, as is the usual case with an amplifying vacuum tube at audible frequencies, this transmission efliciency may readily be shown to be:-

wherein i has its usual operative significance of As will hereinafter appear, L1 and C2 need not both be used, the two being essentially alternative The real component of N, and the similar first term of the real component of N1, are obviously zero if C2 be omitted (i. e., opencircuited, and therefore of a zero capacity value) and it is readily seen that the equation (1) under these conditions becomes:-

This will be found to be inherently an expression for the ratio of the output impedance of the system-i. e., the impedance across which the voltage E2 is taken olf-to the sum of the input impedance-i. e., the impedance across which the input voltage is applied-and the impedance (Ro) out of which the system works.

Returning to the generic expression (l) it has just been noted that the first term of both N and N1 was actually zero if C2 was omitted. On the other hand if C2 be employed but L1 omitted, the two mentioned terms may still be neglected, for the following reason: Both capacities Cz and C1 cooperate with R1, C2 being effective as a signincant shunt reactance at quite high frequencies and C1 as a significant series reactance at quite low frequencies. Because of the contrast of trequencies at which these respective capacities are to be effective C2 is normally very small compared with C1, and the ratio Cz/C1 therefore a very 80 minor fraction, justifying the neglect of the terms in question. This remains true if not only L1 but also C1 be omitted (i. e., C1 theoretically short-circuited) since C1 is then effectively of an inilnite capacity value and the value of the terms in question then actually zero.

For the preferred cases, therefore, ln which only one of the elements L1 and C2 is employed, expression (1) becomes:

: RziR3+.{w[L1+R1R3C2l-1/wC|} E1 R12-ll'Ro+R3+j{[L1+R1(R3+Ro)c2l"' (2) Still further simplification is justified if Re be small compared to R1 and R3; and for purposes 95 of further description of the circuit this will be assumed. 'I'he justifiability of this simplication in any given case, however, must be determined for the particular value of Ro being dealt with. Neglecting it, (2) becomes:-

in which the imaginary components of numerator and denominator are seen to be identical, and the real components to differ only in the presence of the variable Rz in the numerator instead of the xed R1 which appears in the denominator.

Any of the above expressions for transmission efficiency will be seen to be in the form of the quotient of two complex quantities. In accordance with the accepted view that phase relations between different frequency components of sound are not of significance to the ear, these phase relations may be disregarded. The quotient of the effective values of numerator and denominator may therefore be taken as the significant value of any of the expressions. Such effective value of a term is, of course, the square root of the sum of the squares of its real component and of its imaginary component.

Reference being had to expression (3), it will be seen that if L1 be removed by short-circuiting (i. e., assume a zero value), C2 be removed by 125 open-circuiting (i. e., assume a zero value), and C1 be removed by short-circuiting (i. e., assume an infinite value), the circuit is without reactance, the imaginary components of numerator and denominator each become equal to zero, and the transmission efciency becomes independent of frequency. This will be true whatever value the variable R2 be caused to assume by adjustment of the position of slider S. If any of the reactances be restored to circuit, the imaginary components will cease to be zero and will assume a different value for each different frequency. Still, if Rz'is made equal to R1 by positioning of slider S at the superior extremity of the potentiometer, the numerator and denominator are equal, and the transmission eiliciency remains independent of frequency, in that it is unity for all frequencies. But with any of the reaotances in circuit, downward adjustment of the slider, reducing the value of R2 from that of R1, will 145 cause the transmission efliciency to decrease differently for different frequencies-i. e., to changev less, the higher the absolute values of the imaginary components; since with varying R2 the value of the numerator of any of the foregoing expressions obviously changes by a smaller percentage at any frequency, the higher the absolute value of its imaginary component at that frequency.

Thus by making the absolute values of the imaginary components high at the extreme, and low at intermediate, frequencies, the transmission eiciency change occurring upon movement of the slider may be reduced at the extreme frequencies, thereby compensating for the higher sensitivity of the ear to sound amplitude changes at these frequencies. Since the imaginary components take the form of an expression for the reactance of a tuned, or resonant, series circuit, it is very easy so to arrange the values of the reactances that each imaginary component assumes high negative values at low frequencies, a zero value at an intermediate frequency, and high positive values at high frequencies. When L1 is employed, such a resonant circuit is formed by it and the capacity C1. Instead of actually employing such a resonant circuit, however, I may produce its effects by a physically non-resonant circuit arrangement wherein the inductance is absent and its function performed by the capacity C2. This results from the fact that the latter capacity, modified by certain of the resistance values, appears in the imaginary components in a position and algebraic sign similar to those of L1. This similarity of effect, and the opportunity which it affords to replace an inductance by a capacity, I regard as an important and useful detail. of my invention; the capacity is less costly and entirely free of susceptibility to stray magnetic fields; and the physically non-resonant circuit is incapable of producing the objectionable hang-over effects sometimes characteristic of physical resonance. l

It is to be noted 'that if the reduced change in transmission efciency with variation of general volume level be desired at only one end of the audible register instead of at both, this may readily be arranged by appropriately omitting certain of the reactances. Thus if the reduced change be desired only at the high frequencies, the condenser C1 may be omitted and either L1 and/or C2 retained; the imaginary components will then have a positive sign at all frequencies, but will be of appreciable value only at the higher frequencies. Again, if the reduced change in transmission efciency be desired only at the low frequencies, both L1 and C2 may be omitted and.C1 alone retained; the imaginary components will then have a negative sign at all frequencies, but will be of appreciable value only at the lower frequencies. In removing or omitting any of the reactances it is important to observe the method appropriate thereto: i. e., short-circuiting for L1 land C1 and open-circuiting for C2, the appropriate treatment in the formul having been indicated above.

In review of the circuit of Figure 1, it will be seen that I have provided the desired differential effect of the potentiometer volume control on different frequencies, not by parallel resonant circuits shunted from the superior extremity of the potentiometer to the movable contact, but by a series resonant circuit in series with the inferior extremity. I have further provided an alternative, physically non-resonant arrangement for producing identical effects; and also non-resonant arrangements for reducing the action of the control at either end, instead of both ends, of the audible register. One deficiency of the general circuit arrangement of Figure 1 exists, however, and this will hereinafter be seen to be cured in Figure 2. This deficiency comprises the inability to reduce the output volume level to absolute zero,

or below a certain minimum established by the voltage drop in R3, L1 and C1.

In Figure 2 V1 is shown, by way of alternative illustration, as a vacuumtube having a cathode F1, an anode Ai, and a grid to which electric oscillations of sound frequencies may be applied. In the output circuit thereof is shown a conventional coupling resistor Re, and a source B1 of d. c. anode potential, such potential being blocked from the succeeding circuit by conventional blocking condenser C4. For purposes of further analysis the a. c. output voltage of the circuit Vi-Re-Ci-L e., of the tube and its necessary associated apparatus-may be represented by E1. The potentiometer P of Figure 1 is replaced in Figure 2 by potentiometer P', which has thereon a fixed tap T dividing the total resistance thereof into two sections, R1 and R4. A movable contact or slider S may operate over the entire resistance of potentiometer P. That portion of the resistance R1 which may lie below slider S, when the latter is above`tap'T, is designated as R2. The condenser C2 is shunted 'across R1, as in Figure 1; and the resistances R3 and reactances L1 and C1, again in series, are shunted across R4. The value of R4 should be appreciably higher than the impedance, at any frequency to be transmitted, of the series circuit R3-L1-C1 shunting it. If this condition be fulfilled, then as long as slider S is not moved below tap T, the basic action of the potentiometer P and the associated resistance and reactance is sensibly unaltered from that of the circuit of Figure -1, and the formul given above for that circuit may be applied to the circuit of Figure 2 with negligible error. Thus as slider S is moved downward from the superior extremity of the potentiometer, general output volume level Will be reduced and at the same time amplitude ratios between different frequency components will be progressively altered, as in the case of Figure 1. This progressive altering action will continue with downward motion of the slider until the latter has reached the position of tap T. From this point on, continued downward motion will further reduce (eventually to zero) the output sound level, but will not further change the amplitude ratios between diferent frequency components, these ratios remaining fixed at the values they respectively had when the slider was at the position of tap T-i. e., when R2 equalled zero.

While the eifects of various choices of component values is most generically expressed in the formul above, I have tabulated below a list of representative values of components which I have employed to advantage in the circuit of Figure 2; and I have shown as Figure 3 a group or family of curves showing the transmissionfrequency characteristics of the circuit with such component values at various settings of the slider S. It is to be understood that these values and curves are not presented as examples of the best possible compensation for the differential sensitivity of the average earrather do they illustrate the action of my invention, and its adaptability to the production of effects of the nature required for the counteraction of ear characteristics. In these representative values, no L1 is employed, C2 being relied on for the performance of the function performable by L1 and/or C2:-

R1:350.000 ohms C1:.024 microfarads Rs: 20,000 ohms 02:.00015 microfarads R4:200,000 ohms These curves of Figure 3 are plotted to logarithmic scales, -transmission efficiencies being plotted vertically and frequency horizontally. They are computed without regard to the value of R4, the neglect of which is justifiable because of its high value compared to the impedance, at any frequency between 50 and 20,000 cycles, of the series circuit shunting it. Slider S is taken as being so set in the computation of curve A that Rz equals R1; in the computation of curve B, that Rn equals R1/2; of curve C, R1/4; of curve D, R1/8; of curve E, R1/24; in the computation of curve F, that R2 is of zero value; and in the computation of curve G, slider S is taken as being so set that .6 of the resistance of R4 lies below it and .4 above it. Itwill be noted that curve G is identical with curve F, being merely lowered in the logarithmic scale; and it may be shown that the curve for any setting of slider S on resistance R4i. e., below tap T-is identical with curve F, being determined only in position by the exact setting of S. All curves for settings above tap T and thus falling intermediate curves A and F, however, are of respectively different form as well as position, as is illustrated by B, C D and E.

It may readily be seen from the curves that the imaginary components of numerator and denominator of (3) became zero at approximately 1,000 cycles, the relative values of the reactances having been chosen to that end. But an infinite number of sets of reactance values would produce such zero value at such frequency. It is therefore appropriate to say that the absolute values of the reactances (in relation to the resistances and particularly to'R1) aifect the steepness of the rise of each of the lower curves on either side of its low point. Lower C1 and higher C2 capacity values (and higher values of inductance L1 if employed) tend to produce steeper slopes in the curves, and vice versa. These reactance Values (relative to resistance) affect the steepness of the curves essentially because they determine the curves themselves at and toward the extremities; and it is to be noted that extreme reactance values will not only steepen the slopes of the curves, but will also flatten off the extremities. A secondfactor affecting the steepness of the slopes is the resistance Rs, which (in relation to R1) essentially xes the central portions of the lower curves. These central portions become lower, and the slopes thus steeper, the smaller the value of R3, and vice versa. Of course, the value of R3 also modifies the action 'of the capacity C2, in that C2 appears in the imaginary components multiplied by R3. In order, therefore, that the eifect of changing R3 be confined to a change merely of the form of a curve, the value of C2 should be varied approximately inversely to the variation of Rs.

If the right-hand (high frequency) rise, or slope, of each of the lower curves shown in Figure 3 be replaced by a horizontal line at the level of the central portion of the curve, and the lefthand slope be made to curve somewhat more gradually into such horizontal line, the resulting curves will be characteristic of the system with C: (and L1) omitted. Conversely, if similar changes be made, but reversed between rightand termine the other extremity of, and generally the position in the frequency spectrum of the slope of, each curve. Y

The element L1 having been omitted in the example of component values given above and in the curves, compiled therefrom, I have included.

as Figure 4 of the drawing the essential portion of Figure 2 but with the element L1 omitted in manner as above discussed.

It is desirable that vthe resistance element of potentiometer P', comprising resistances R1 and R4, be specially tapered if relatively uniform change, either logarithmic or arithmetic, of average output volume level be desired for equal distances of motion of slider S. Such taper may be readily determined by trial with the particular values of the components chosen for use in any given case.

If the circuit of Figure 2 be employed with C2 as the sole reactance, not only will L1 and C1 be omitted (theoretically short-circuited), but the resistance R3 may also be omitted (theoretically open-circuited), so that no circuit or element shunts R4; but in this case the actual value of R4 should be used in the formul as a value of R3 therein. In this case R4, in addition to performing its usual function of permitting reduction of volume level to zero, is also performing the normal function of R3, and should assume a value indicated as desirable for the latter.

It will be appreciated not only that wide departures may be made from the component values set forth above as typical, but also that various modifications may be made from the precise circuits and arrangements described. Again, while I have most completely described my invention in connection with-the problem of maintaining apparent output tone quality approximately constant at various volume levels, it will be understood that it is equally useful for the production of a wide variety of tone quality changes coincident to volume level change. Such and other modifications and uses will not necessarily constitute departures from the spirit or scope of my invention, as hereinabove set forth and in the appended claims dened.

It will of course be understood that the transmission eiiiciency of theV systems herein disclosed at any frequency and with any adjustment is the reciprocal of the attenuation effected by the system at such frequency and with such adjustment; and that the transmission-frequency characteristic with any adjustment is the inverse of the attenuation-frequency characteristic with such adjustment. Except as specially qualified, however, the term attenuation is used herein in the broader context of general or average reduction of amplitude or volume level at all frequencies, independent of the degree of non-uniformity at the several frequencies.

1. The combination with a source of plural frequency sound-representing electric oscillations, of an adjustable attenuating system therefor comprising a resistive voltage-dividing circuit; means for applying said oscillations thereacross; reactive means xedly bridged across a Lacasse portion of said circuit; output connections from said circuit; and means for controlling the attenuation eiected by and the transmission-frequency characteristic of said system in predetermined mutual relationship, said means consisting in a single contact selectively movable over said circuit to a plurality of positions within and to a plurality of positions without said bridged portion thereof, said contact forming one of said output connections.

2. The combination with a source of plural frequency sound-representing electric oscillations, of an adjustable attenuating system therefor comprising a resistive voltage-dividing means; an input circuit connected thereacross for applying said oscillations thereto; a contact substantially continuously movable over said voltage-dividing means; an output circuit connected from said contact to an extremity of said voltage-dividing means; and a reactive system permanently bridged to said input circuit from a point on said voltage-dividing means intermediate the extremities of the motion range of said contact; whereby adjustment of said contact simultaneously controls the attenuation effected by and the transmission-frequency characteristic of said system in predetermined mutual relationship.

3. The combination with a source of plural frequency sound-representing electric oscillations, of an adjustable attenuating system therefor comprising a resistive element and an input circuit connected thereacross for applying said oscillations thereto; a contact associated with said element and anl output circuit connected from said contact to an extremity of said element, said contact being substantially continuously movable over said element to vary said attenuation at all frequencies; a fixed tap on said element intermediate the extremities of the motion range of said contact; and xed reactive means, bridged from said fixed tap to said input circuit, for progressively augmenting said attenuation at intermediate frequencies as said attenuation at all frequencies is increased over a iinite range.

4. The combination with a source of plural frequency sound-representing electric oscillations, of an adjustable attenuating system therefor comprising a potentiometer including a resistive element, a iixed tap thereon, and a contact movable thereover to any of a plurality of positions on each side of said tap; a circuit, including reactance, connected between said tap and an extremity of said resistive element; means for applying said oscillations across said resistive element; and means for transmitting oscillations appearing between an extremity of said resistive element and said movable contact; whereby adjustment of said contact simultaneously controls the attenuation eiected by and the transmission-frequency characteristic of said system in predetermined mutual relationship.

5. The combination with a source of plural frequency sound-representing electric oscillations, of an adjustable attenuating system therefor comprising a potentiometer formed by a resistive element and a contact substantially continuously movable thereover; a circuit, including reactance, having a first extremity connected to a point on said resistive element intermediate the extremities of the motion range of said contact and a second extremity connected to an extremity of said element; means for applying said oscillations across said resistive element; and means for transmitting oscillations appearing between said extremity of said element and said movable contact; whereby adjustment of said contact simultaneously controls the attenuation effected by and. the transmission-frequency characteristic of said system in predetermined mutual relationship.

6. The combination with a source of plural frequency sound-representing electric oscillations, of an adjustable attenuating system therefor comprising a potentiometer including a resistive element, a iixed tap thereon, and a contact movable thereover to any of a plurality of positions on each side of said tap; a series circuit, including at least one reactance and resistance, connected between said tap and an extremity of said resistive element; means for applying said oscillations across said resistive element; and means for transmitting oscillations appearing between said extremity of said element and saidmovable contact; whereby adjustment of said contact simultaneouslycontrols the attenuation eii'ected by and the transmission-frequency characteristic of said system in predetermined mutual relationship.

7. The combination with a source of plural frequency sound-representing electric oscillations, of an adjustable attenuating system 4therefor comprising a resistive element; a circuit, including reactance, having a first extremity connected to an extremity of said element; means for applying said oscillations across said element and said circuit in series; a further resistive element connected across said circuit; a single contact consecutively movable over said elements to any of a plurality of positions on each; and means for transmitting oscillations appearing between said movable contact and the second extremity of said circuit; whereby adjustment of said contact lsimultaneously controls the attenuation effected by and the transmission-frequency characteristic of said system in predetermined mutual relationship.

8. In a potentiometer system for the adjustable attenuation of plural frequency sound-representing electric oscillations, the combination of means included in a portion of said system for increasing the impedance of such portion progressively as frequency approaches a limit of audibility; a further portion of said system connected in series with said rst mentioned portion; means for applying said oscillations across said serially connected portions; means for transmitting oscillations from said serially connected portions; and a single continuously adjustable means operable selectively to reduce said transmisison from said further portion, or to eliminate the same and reduce the transmission from said first mentioned portion; whereby adjustment of said last mentioned means simultaneously controls the attenuation effected by and the transmission-frequency characteristic of said system in predetermined mutual relationship.

9. In a potentiometer system for the adjustable attenuation of plural frequency sound-representing electric oscillations, the combination of means included in a portion of said system for increasing the impedance of such portion progressively as frequency approaches a limit of audibility; a further portion of said system connected in series with said rst mentioned portion, said further portion having a substantially constant impedance with varying frequency; means for applying said oscillations across said serially connected portions; means for transmitting oscillations from said serially connected portions; and a single voltage-reducing means successively adjustable within said further and said rst mentioned portions to reduce said transmission from said further portion or to eliminate the same and reduce the transmission from said first mentionedportion; whereby adjustment of said last mentioned means simultaneously controls the attenuation e'ected by and the transmission-frequency characteristic of said system in predetermined mutual relationship.

10. The combination with a source of plural frequency sound-representing electric oscillations, of an adjustable attenuating system therefor comprising a potentiometer including a resistive element, a contact substantially continuously movable thereover, and a tap thereon intermediate the extremities of the range of motion of said contact; means for applying said oscillations across said resistive element; means for transmitting oscillations appearing between said contact and an extremity of said element; and a series circuit resonant to an intermediate frequency connected between said tap and said extremity of said element; whereby adjustment of said contact over the portion of said element between the other extremity thereof and said tap effects a greater change of attenuation at intermediate than at extreme frequencies.

11. In a potentiometer system for the adjustable attenuation of plural frequency sound-representing electric oscillations, the combination of means included in a portion of said system for increasing the impedance of such portion progressively with increasing frequency difference from a mean frequency; a further portion of said system connected in series with said first mentioned portion, said further portion having a substantially constant impedance with varying frequency; means for applying said oscillations across said serially connected portions; means for transmitting oscillations from said serially connected portions; and a single voltagereducing means successively adjustable within said further and said rst mentioned portions to reduce said transmission from said further portion or to eliminate the same and reduce the transmission from said first mentioned portion; whereby adjustment of said last mentioned means within said further portion effects a smaller change of attenuation at extreme than at intermediate frequencies.

12. The combination with a source of plural frequency sound-representing electric oscillations, of an adjustable attenuating system therefor comprising a potentiometer formed by a resistive element and a contact substantially continuously movable thereover; a capacity connected between a rst extremity of said resistive element and a point thereon intermediate the extremities of the range of motion of said contact; means for applying said oscillations across said resistive element; and means for transmitting oscillations appearing between the second extremity of said resistive element and said movable contact; whereby adjustment of said contact over the portion of said element shunted by said capacity effects a smaller change of attenuation at high frequencies than at other frequencies.

13. The combination with a source of plural frequency sound-representing electric oscillations having output impedance R0, of an adjustable attenuating system for said oscillations,

said system having fixed resistive parameters R1 and R3 serially connected, an adjustable resistive parameter R2 forming part of R1, a relatively large capacitive parameter C1 in series with R3, and a relatively small capacitive parameter C2 shunting R1, the transmission eiliciency of said system in said combination being given substantially by the following expression:

and the values of said xed parameters being so chosen that the imaginary component of the numerator in the foregoing expression assumes a zero value at an intermediate frequency; whereby adjustment of the value of R2 effects a smaller change of oscillation attenuation at extreme than at intermediate frequencies.

14. 'Ihe combination with a source of plural frequency sound-representing electric oscillations, of an adjustable attenuating system therefor comprising a potentiometer including a resistive element, a fixed tap thereon, and a contact movable thereover to any of a plurality of positions on each side of said tap; a series circuit, comprising a capacity and a resistance, connected between said tap and a rst extremity of said resistive element; a smaller capacity connected between said tap and the second extremity of said resistive element; means for applying said oscillations across said element; and means for transmitting oscillations appearing between said first extremity of said element and said movable contact; whereby adjustment of said contact over a portion of said resistive element effects a smaller change of attenuation at exl treme than at intermediate frequencies.

15. In the amplitude control of plural audio frequency electric oscillations by a potentiometer 113 system whose transmission eciency at any frequency is substantially the ratio of its output impedance to the sum of its input impedance and the impedance out of which the system Works, the method of volume control of sound translated from said oscillations to provide volume changes of apparently similar degree at different frequencies, which consists in varying the output impedance of said system by greater percentages at intermediate than at extreme frequencies, while maintaining essentially unchanged the input impedance of said system at each discrete frequency.

16. In the amplitude control of plural audio frequency electric oscillations by a potentiometer system whose transmission eciency at any frequency in the lower audio band is substantially the ratio of its output impedance to the sum of its input impedance and the impedance out of which the system works, the method of volume 135 control of sound translated from said oscillations to provide volume changes of apparently similar degree at various frequencies within said band, which consists in varying the output impedance of said system within said band by perl centages which are progressively smaller the lower the frequency, while maintaining essentially unchanged the input impedance of said system at each discrete frequency.

CHARLES T. JAcoBs. 

